1. Field of the Invention
This invention relates to an image processing apparatus which expresses an image according to generation and non-generation of dots and more particularly to an image processing apparatus which expresses an image approximately in tones by controlling generation of dots based on a multi-tone image signal.
2. Description of the Prior Art
There is widely used a technique of image expression in which an input image signal is converted into a binary signal and dots are generated in accordance with the binary signal. In this case, intermediate tones are expressed by degree of density of generated (shaded) dots. Regarding a method of binary-coding the image signal for the reproduction of intermediate tones of the input image, attention is recently drawn to the error diffusion technique, as disclosed in Japanese Patent unexamined Publications JP-A-62-242473, JP-A-63-288567, JP-A-63-217768, JP-A-63-214073, JP-A-1-97066, and JP-A-1-115272.
The error diffusion technique is intended to calculate the difference of concentration between the input image and output image on a pixel-by-pixel basis, and the resulting error component is diffused with specific weights to adjacent pixels.
In a case that a pixel of the input image, which is generally square, needs to be expressed by a dot of other shape (e.g. for example, circle), the dot is generally dimensioned to include the corresponding pixel completely. In consequence, when such a dot is generated for a certain pixel, the dot should overlap with an adjacent pixel.
The overlap of dots affects the density of the output image. For example, when a white dot (actually no dot) is printed next to a black dot, the white dot has its area reduced due to the jut-out area of the black dot, resulting in a blackish tone of this portion. Similarly, when a black dot is printed next to a white dot, the black dot juts out into the white dot area, resulting in a blackish tone of this portion. The binary-coding process based on the conventional error diffusion technique is incapable of correcting the influence of dot overlap on the density of the output image.